1. Field of the Invention
This invention relates to metalorganic chemical vapor deposition (MOCVD) of strontium bismuth tantalate and in particular to a tetrahydrofuran-based solvent system for precursor delivery for MOCVD of such material
2. Description of the Related Art
Liquid delivery MOCVD is the preferred method for depositing thin films of ferroelectric materials because of the ability to carefully control composition and deposit conformal films of high density. In such processes a solution or suspension of precursors (metalorganic source reagents for the respective metal components of the product film material) is vaporized, preferably at high rate by xe2x80x9cflash vaporizationxe2x80x9d techniques to produce a corresponding precursor vapor which may be mixed with carrier and/or additive gases (e.g., oxygen-containing gas, inert gases such as argon, helium, etc., co-reactive gases, diluents, etc.) to form a vapor mixture. The vapor mixture then is flowed to a deposition zone where the precursor mixture is contacted with a substrate at elevated temperature to effect deposition from the vapor phase onto the substrate of a desired material. MOCVD of SrBi2Ta2O9 (SBT) and related materials use precursors that are chemically compatible in solution for long periods of time and also have similar decomposition characteristics vis-xc3xa1-vis one another in the MOCVD process. Sr(thd)2-LBA, Bi(thd)3, and Ta(O-i-Pr)4(thd) (where thd stands for 2,2,7,7-tetramethyl-3,5-heptanedionato, LBA stands for a Lewis base adduct and O-i-Pr stands for isopropoxide) provide a preferred combination of precursors for such purpose. Precursors for deposition of SBT and other ferroelectric materials, and their use in liquid delivery MOCVD formation of high quality product films, are more fully described in U.S. patent application Ser. No. 08/960,915 filed Oct. 30, 1997 in the names of Thomas H. Baum, et al., now issued as U.S. Pat. No. 5,859,274, and U.S. patent application Ser. No. 08/976,087 filed Nov. 20, 1997 in the names of Frank S. Hintermaier, et al.
The best previously known solvent system for this precursor suite (of Sr(thd)2-LBA, Bi(thd)3, and Ta(O-i-Pr)4(thd)) consisted of alkanes and in particular a mixture of octane, decane, with an excess of the LBA of the strontium precursor, Sr(thd)2. In particular, a ratio of 5 parts octane, 4 parts decane, and 1 part LBA was found to maintain the precursors in a stable condition and deliver them reliably to a flash vaporization equipment. Solvent compositions of such type are more fully described in U.S. patent application Ser. No. 08/975,372 filed Nov. 20, 1997 in the names of Thomas H. Baum, et al., now issued as U.S. Pat. No. 5,916,359.
However, the alkane-based solvents have limited solubility of the aforementioned SBT precursors, especially for the Sr(thd)2, which only dissolves to about 0.2 M in a 5:4:1 mixture of octane:decane:pmdeta. This limited solubility characteristic of the solvent composition has disadvantages for liquid delivery vaporization and the subsequent deposition process. During vaporization, wherein the precursor solution typically is flowed through a liquid delivery tube to a heated surface, the molarity of the precursor solution must remain significantly below the solubility limit to prevent precipitation of solid metalorganic particles from the solution, which would otherwise occur as solvent boils off in the liquid delivery tube, and which would lead to clogging of the liquid delivery tube. In addition, low solution molarity requires the vaporization of more solvent in order to deliver the same amount of precursor to the process. This creates undue requirements for the supply of heat to the metalorganics in the vaporizer, since more heat must then be provided to volatilize the additional solvent that is present.
The deposition process itself also is adversely affected by low molarity solutions. By way of example, film growth rates were limited to 3-4 nanometers per minute (nm/min) in a prototype MOCVD reactor delivering a 0.30M solution at a rate of 0.2 ml/min. Growth rates should be 2-10 times higher than this level in order to meet the requirements of a xe2x80x9cmanufacturable process.xe2x80x9d In addition, limiting the rate of delivery of precursor to the substrate surface makes it more difficult to push the process into a surface kinetic-controlled growth regime where conformality to non-planar structures is improved, relative to film formation processes operating outside of such regime.
It would therefore be an advance in the art to provide a solvent medium for liquid delivery MOCVD of precursor compositions such as SBT, which overcome the aforementioned difficulties.
The present invention relates to a precursor composition that is usefully employed for liquid delivery MOCVD applications, e.g., for SBT, doped SBT and other ferroelectric precursors.
The invention relates in one aspect to a precursor composition useful for liquid delivery MOCVD, comprising SBT precursors dissolved in a solvent system containing tetrahydrofuran (THF). The solvent system may for example be constituted in major portion by THF, or it may be constituted solely by THF, or it may consist essentially of THF. In such precursor composition, Sr(thd)2(THF)4 is a preferred chemical species for the introduction of Sr.
The invention relates in another aspect to a precursor composition useful for liquid delivery MOCVD comprising SBT precursors dissolved in a solvent system comprising 90-99 parts by volume tetrahydrofuran and 1-10 parts by volume Lewis base ligand (LBA). In such composition, Sr(thd)2(LBA)x, wherein x is from 1 to 4, is a preferred chemical species for the introduction of Sr.
The invention relates in another aspect to a precursor composition useful for liquid delivery MOCVD, comprising SBT precursors and a dopant precursor, e.g., niobium beta-diketonate, dissolved in a solvent system including tetrahydrofuran. In such composition, Sr(thd)2(THF)4 is a preferred chemical species for the introduction of Sr.
The invention relates in a still further aspect to a precursor composition useful for liquid delivery MOCVD, comprising SBT precursors and a dopant precursor, e.g., niobium beta-diketonate, dissolved in a solvent system comprising 90-99 parts by volume tetrahydrofuran and 1-10 parts by volume LBA. In such composition, Sr(thd)2(LBA)x wherein x is from 1 to 4, is a preferred chemical species for the introduction of Sr.
Another aspect of the invention relates to an SBT precursor composition comprising SBT precursors dissolved in a solvent medium including tetrahydrofuran, e.g., a solvent medium consisting essentially of tetrahydrofuran, with such SBT precursor composition having a boiling point at 1 atmosphere pressure of about 66xc2x0 C. The SBT precursors of such composition desirably comprise Sr(thd)2(THF)4 as a preferred chemical species for the introduction of Sr due to its high solubility and low melting point.
Another aspect of the invention relates to an SBT precursor composition comprising SBT precursors and a dopant component, e.g., a dopant precursor, dissolved in a solvent medium containing tetrahydrofuran, having a boiling point at 1 atmosphere pressure of about 66xc2x0 C. The SBT precursors of such composition desirably comprise Sr(thd)2(THF)4 as a preferred chemical species for the introduction of Sr due to its high solubility and low melting point.
A further aspect of the invention relates to a method of forming SBT material on a substrate, comprising:
providing a precursor composition for SBT;
volatilizing the precursor composition to yield a precursor vapor; and
contacting the precursor vapor with the substrate to deposit SBT thereon;
wherein the precursor composition comprises SBT precursors dissolved in a solvent medium containing tetrahydrofuran. The SBT precursors of such composition desirably comprise Sr(thd)2(THF)4 as a preferred chemical species for the introduction of Sr due to its high solubility and low melting point.
A further aspect of the invention relates to a method of forming doped SBT material on a substrate, comprising:
providing a precursor composition for doped SBT;
volatilizing the precursor composition to yield a precursor vapor; and
contacting the precursor vapor with a substrate to deposit doped SBT thereon.
In such method, the precursor composition advantageously comprises SBT precursors and a dopant precursor, e.g., niobium beta-diketonate, dissolved in a solvent medium containing tetrahydrofuran, wherein Sr(thd)2(THF)4 is a preferred chemical species for the introduction of Sr due to its low melting point and high solubility in THF.
A further aspect of the invention relates to a method of forming SBT material on a substrate, comprising:
providing a precursor composition for SBT;
volatilizing the precursor composition to yield a precursor vapor; and
contacting the precursor vapor with a substrate to deposit SBT thereon, wherein the precursor composition comprises SBT precursors dissolved in a solvent system comprising 90-99 parts by volume tetrahydrofuran and 1-10 parts by volume LBA. In such composition, Sr(thd)2(LBA)x, wherein x is from 1 to 4, is a preferred chemical species for the introduction of Sr.
In another aspect, the invention relates to a method of forming an SBT film on a substrate, comprising liquid delivery MOCVD using a precursor composition comprising SBT precursors dissolved in a solvent medium containing tetrahydrofuran. In such composition, Sr(thd)2(THF)4 is a preferred chemical species for the introduction of Sr due to its low melting point and high solubility in THF.
In a further aspect, the invention relates to a method of forming a doped SBT film, e.g., niobium doped SBT, on a substrate, comprising liquid delivery MOCVD using a precursor composition comprising SBT precursors and a dopant precursor dissolved in a solvent medium containing tetrahydrofuran. In such composition, Sr(thd)2(THF)4 is a preferred chemical species for the introduction of Sr due to its low melting point and high solubility in THF.
A still further aspect of the invention relates to a method of forming an SBT film on a substrate, comprising liquid delivery MOCVD using a precursor composition comprising SBT precursors dissolved in a solvent system comprising tetrahydrofuran and a Lewis base ligand. In such composition, Sr(thd)2(LBA)x wherein x is from 1 to 4, is a preferred chemical species for the introduction of Sr.
The aforementioned precursor compositions of the present invention provide preferred combinations of precursors that are chemically compatible in solution for extended periods of time and have similar decomposition characteristics vis-à-vis one another in the MOCVD process for deposition of SBT and other ferroelectric materials.
Other objects, features and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.